1. Technical Field
The invention relates to a micro-probe apparatus. In particular, the invention relates to an ultra-small, conductive micro-probe device that is suited for measuring, detecting, or transmitting electrical signals.
2. Description of the Prior Art
Conventional scanning probe microscopes (SPMs), for example scanning tunneling microscopes (STMs) and scanning atomic force microscopes (AFMs), have been primarily used to measure surface shapes. Recently, however, such devices have been widely used to measure various physical quantities of microscopic areas of an object. When measuring such physical quantities with a measurement device such as an SPM, particularly when measuring the electrical characteristics of a measurement target, an electrical connection is made between the measurement device and the measurement target using an extremely fine micro-probe apparatus.
A conventional conductive micro-probe device, for example as is typically used in an STM, includes a micro-probe formed as a thin film on the surface of a cantilever support member that is made from a simple metal element, such as Pt, Ir, or Au. The micro-probe comprises a fine rod formed on the cantilever support member and that is made of a single metal, such as tungsten (W). Such micro-probe includes a micro-probe tip that is formed or otherwise produced on the end of the micro-probe and that is tapered to a point, for example, by etching. The micro-probe tip is typically tapered such that it has a triangular pyramid shape or a quadrilateral pyramid shape. The micro-probe tip is typically made of such materials as silicon (Si) or silicon nitride (Si.sub.3 N.sub.4), and may have a radius of curvature of about 50 nm.
Deformation or wear of a conventional micro-probe tip may occur as the micro-probe tip moves while in contact with the measurement target, for example, when the micro-probe device is used to measure the physical quantities of the target measurement surface. Such deformation or wear may result in various problems with the electrical and mechanical characteristics of the micro-probe device. Thus, a major factor in determining the useful life of equipment that is used to perform such measurements is the deterioration of the micro-probe tip contained therein.
The diameter of the area of contact between the measurement target and the micro-probe tip is measured in terms of nanometers. The useful life of the micro-probe tip is reduced because the volume of the micro-probe tip necessarily becomes smaller as the micro-probe tip wears during use, even though the amount of such wear is only microscopic.
The actual useful life of the micro-probe device is affected by such factors as the specified ranges of acceptable values that may be produced by the micro-probe device, e.g., the tolerance of the micro-probe tip, and the expected number of times that actions involving the micro-probe tip, such as contact operations, are possible while the micro-probe tip is otherwise within such specified ranges. It should be appreciated that the useful life of the micro-probe tip may be shortened by any one, or a combination of such factors.
For example, when a micro-probe device is used in the read head of an ultra-small, large capacity memory device, it is critical that a specific area of the micro-probe device be in contact with the measurement target, e.g., the storage medium. In such application, such critical factors as electric charge, resistance, current, and voltage must be precisely measured by the micro-probe device. Unfortunately, a conventional micro-probe device having, for example, a quadrilateral pyramid-shaped micro-probe tip is unacceptable for such application because the contact area between the micro-probe tip and the measurement target necessarily increases as the micro-probe tip becomes worn. As a result, the size of the area on the surface of the measurement target that stores a unit of data must be increased to accommodate the increase in contact area between the micro-probe tip and the surface of the measurement target resulting from such wear.
In an ultra-small, large capacity memory device, for example, as is disclosed by Saitoh et al. in Japanese unexamined patent no. 8-115600, the permitted contact area between the micro-probe tip and the surface of the measurement target is not readily altered to accommodate an increase in contact area due to wear of the micro-probe tip because the micro-probe tip is the end of a cylindrical micro-probe that has a uniform cross sectional area. The micro-probe used in Saitoh et al. is made from a single conductive material, such that wear of the micro-probe tip is a major problem.
Furthermore, if the micro-probe device deteriorates to the point that its useful life is at an end, then the useful life of the memory device itself is also at an end because the memory device is typically fabricated in such way that it is not convenient to replace a worn micro-probe device with a new micro-probe device. Thus, the useful life of the micro-probe device directly affects the useful life of the memory device. It would therefore be advantageous to provide a micro-probe device having a longer useful life.